Presently, in the 3GPP (3rd Generation Partnership Project), the standardization is advancing with respect to a W-CDMA (Wideband-Code Division Multiple Access) system which is one of third-generation mobile communication systems. Moreover, as one of themes of the standardization, there is specified an HSDPA system which provides a large-amount and high-speed packet data transfer (up to approximately 14 Mbps) in a down direction (downlink).
The HSDPA employs an adaptive coding and modulation technique and has one feature that the switching between a QPSK (Quadrature Phase Shift Keying) modulation technique and a 16 QAM (Quadrature Amplitude Modulation) technique is made in accordance with radio environment between a base station and a mobile station. Moreover, the HSDPA employs an HARQ (Hybrid Automatic Repeat request) technique. That is, the HSDPA features that, when a mobile station has detected an error with respect to received data from a base station, the data is retransmitted from the base station in response to a request from the mobile station and the mobile station carries out the error correction decoding by use of both the previously received data and the retransmitted received data. Thus, the HARQ utilizes the previously received data effectively even if an error exists, thereby enhancing the error correction decoding gain and suppressing an increase in the number of times of retransmission.
Among the principal radio channels for use in the HSDPA, there are HS-SCCH (High Speed-Shared Control Channel), HS-PDSCH (High Speed-Physical Downlink Shared Channel) and HS-DPCCH (High Speed-Dedicated Physical Control Channel).
Both the HS-SCCH and HS-PDSCH are a common channel in a down direction (that is, direction from a base station to a mobile station), and the HS-SCCH is a control channel for transmitting various types of parameters on data through the HS-PDSCH. For example, the parameters include information such as modulation type information indicative of which of modulation techniques is used for transmitting data through the HS-PDSCH, the number of spread codes to be allocated and a pattern of rate matching to be made with respect to transmission data.
On the other hand, the HS-DPCCH is an individual control channel in an up direction which is a direction from a mobile station to a base station, and it is used in a case in which the mobile station transmits an ACK signal or NACK signal to the base station in accordance with the reception result of data received through the HS-PDSCH. If the mobile station has failed to receive the data (a case in which a CRC error occurs in the received data, or in other cases), the NACK signal is transmitted from the mobile station, and the mobile station executes the retransmission control).
In addition, the HS-DPCCH is also used when a mobile station which has measured a reception quality [for example, SIR (Signal-to-Interference Ratio)] of a received signal from a base station transmits the measurement result as CQI (Channel Quality Indicator) to the base station. The base station, based on the received CQI, determines the quality of the radio environment in the downlink. If the result of the determination indicates a satisfactory radio environment, the base station conducts the switching to a modulation technique which enables the transmission of data at a higher speed, while conversely, if the result of the determination does not indicate a satisfactory radio environment, the base station carries out the switching adaptively to a modulation technique which transmits the data at a lower speed.
Meanwhile, in the HSDPA technique, a large number of unspecified users share a limited HSDPA dedicated channel, and there is a need to control the power distribution between the shared channel and individual channels for each user.
In this case, the high-speed packet data communication quality (throughput) of the HSDPA depends upon the HSDPA transmission electric energy, the retransmission control, the adaptive modulation allocation and others. Of these, an important object in the improvement of the throughput is the transmission power which the HSDPA occupies. Moreover, although a plurality of users are accessible at the same time, an increase in the number of users causes a decrease in the power to be allocated to each user, which leads to a reduction of the throughput.
In the recent years, the interference suppression effect is expectable by forming a directional beam by use of an adaptive array antenna technology or the like, which enables the enhancement of the cell capacity or the reduction of the transmission power needed for assuring a given communication quality. The adaptive array antenna is a technique capable of increasing the channel capacity, and it can form a beam adaptively to an intending user and can set a null with respect to a user who becomes a large interference source.
That is, by forming a beam in a direction toward the target user and directing a null in a direction of the user who becomes a large interference source, it is possible to receive a radio signal with high sensitivity from the target user and avoid the reception of a radio signal from the large interference source. This can reduce the degree of interference, which leads to increasing the channel capacity.
Although the improvement of the throughput is expectable by applying this technique to the HSDPA, there are the following problems.
That is, since the HS-SCCH is a control channel shared by a plurality of users, it is difficult to form a directional antenna beam for each individual user. For this reason, in the HS-SCCH, there is a need to make the directional beam constant. For example, the following Patent Document 1 proposes that a different HS-SCCH code is allocated arbitrarily to each beam.    Patent Document 1: Japanese Patent Laid-Open No. 2004-297750